Tell us something more, what error you get...
Do you have one of latest CWM (like version 6.0.4.5)? (What ROM are you using now?) If not, try installing CM 10.2 first and then 4.4 ROM.
PS. If you have Android =< 4.2 you will get status 7 error on first try when installing Android 4.3+. So after error you have to 'reboot to recovery' and install the ROM again.
Follow this:
Reboot into Recovery
Wipe data/factory reset
Wipe cache partition
Advanced > Wipe dalvik cache
Main menu
Install zip from sdcard
Choose zip from sdcard(internal)
Select your rom zip and confirm
Your phone at this moment may reboot automatically and come into a bootloop or show status 7 error or set_metadata_error so you need to flash the rom again.
Remove battery and reboot into new recovery and then flash the zip.file after that again reboot into recovery wipe everthing and flsah your rom and reboot
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Then you could try to revert to stock Gb 2.3.6 using Odin and then use the methods described above by AreXe or OmreJ to update 2.3 > 4.4
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